The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Cooling systems have applicability in a number of different applications where a fluid is to be cooled. The fluid may be a gas, such as air, or a liquid, such as water. Example applications are heating, ventilation, air conditioning (HVAC) systems that are used for cooling spaces where people are present such as offices and data center climate control systems. A data center may refer to a room having a collection of electronic equipment, such as computer servers.
In FIG. 1, an air conditioner 50 that may be used in, for example, a computer room is shown. The air conditioner 50 includes a cooling circuit 51 and a cabinet 52. The cooling circuit 51 is disposed in the cabinet 52 and includes an evaporator 54, an air moving device 56, a compressor 58, a condenser 60, and an expansion valve 62. The evaporator 54, compressor 58, condenser 60 and expansion valve 62 are connected in a closed loop in which a cooling fluid (e.g., phase change refrigerant) circulates. The evaporator 54 may include a V-coil assembly with multiple cooling slabs to provide increased cooling capacity. The evaporator 54 receives the cooling fluid and cools air passing through openings in evaporator 54. The air moving device 56 (e.g., a fan or squirrel cage blower) draws the air from an inlet (not shown) in the cabinet 52 and through the evaporator 54. The cooled air is directed from the evaporator 54 and out a plenum 64 in the cabinet 52.
The compressor 58 circulates the cooling fluid through the condenser 60, the expansion valve 62, the evaporator 54 and back to the compressor 58. The compressor 58 may be, for example, a scroll compressor. A scroll compressor may be a fixed capacity or digital variable capacity compressor. A scroll compressor typically includes two offset spiral disks. The first spiral disk is a stationary disk or scroll. The second spiral disk is an orbiting scroll. The cooling fluid is received at an inlet of the scroll compressor, trapped between the offset spiral disks, compressed, and discharged at a center (or outlet) towards the condenser 60. The condenser 60 may be a micro-channel condenser that cools the cooling fluid received from the compressor 58. The expansion valve 62 may be an electronic expansion valve and expand the cooling fluid out of the condenser 60 from, for example, a liquid to a vapor.
Although a single cooling circuit is shown in FIG. 1, multiple cooling circuits may be included and arranged in series to provide multiple stages of cooling. The cooling circuits may minimize energy consumption by reducing pressure ratios of the compressors. A pressure difference of a compressor refers to a difference between an inlet or suction pressure and an outlet or head pressure of the compressor. To increase operating efficiencies, the pressure differences may be further reduced based on established room conditions. The decrease in the pressure differences can lead to the stationary scrolls of the compressors separating from the orbiting scrolls. This results in unpredictable and undirected “unloading” of the compressors. A compressor is unloaded when the compressor is in a minimal vapor displacement condition (or transfers a minimal amount of fluid). The unloading of the compressors reduces the cooling capacity and temperature control stability of the cooling circuits.
At elevated return air temperatures (90+° F./32.2+° C.) compressor efficiency gains can be maximized. At these temperatures and when, for example, Refrigerant 410A is used and is at steady-state, operating pressure can result in undirected unloading of the scroll compressors.
An air conditioning system may include one or more compressors including a tandem set of compressors. The tandem set may include two or more compressors of equal or unequal volumetric displacement. A first compressor may be a digital pulse width modulation (PWM) scroll compressor that receives a PWM percentage signal to control capacity of the first compressor. As an alternative, the first compressor may be any variable capacity scroll compressor that receives a proportional percentage signal to control compressor speed. A second compressor may be a fixed capacity scroll compressor with simply ON/OFF capacity control. Additional compressors may be included in the tandem set. The additional compressors may be digital PWM scroll compressors, variable speed scroll compressors, and/or fixed capacity scroll compressors. Suction and discharge lines of the compressors in the tandem set may be piped in parallel to form the tandem set.
The tandem set offers an energy efficient configuration by allowing the digital PWM scroll compressor to be activated prior to the fixed capacity scroll compressor. This effectively allows the tandem set to provide partial-displacement operation with a reduced volumetric displacement/capacity until additional capacity is needed from the fixed capacity scroll compressor. While this partial-displacement operation is efficient, providing this operation in conjunction with a condenser at low ambient temperatures and an evaporator at high ambient temperatures can cause prolonged operation at a minimum compressor pressure difference. This low-pressure difference can also cause an unloaded compressor condition, which can lead to reduced cooling capacity of the compressors.
The unloaded compressor condition at startup is prevalent at low-outdoor ambient temperatures (e.g., temperatures less than 40° F./4.4° C.). As an outdoor ambient temperature descends and an incoming air temperature to the condenser is reduced, the duration of unloaded time may be increased.
The compressor configuration of the tandem set also allows for energy efficient temperature control by providing a wide range of capacity modulation for a cooling circuit of an air conditioning system. However, when an incoming load to the tandem set is reduced such that the fixed capacity scroll compressor is deactivated, a rate of volumetric displacement for the tandem set can be reduced by at least 50%. This can cause the pressure difference of the tandem set to temporarily decrease to a low-pressure difference. When the pressure difference is less than a predetermined pressure difference, the suction pressures and the head pressures of the compressors are approximately equal to each other, resulting in an unloaded condition. This reduces the ability of the tandem set to move vapor, which reduces cooling capacity.